In general, an electric capacitive displacement sensor is a device for producing an electric signal indicating a variation in capacitance corresponding to a displacement between two conductive plates. When the two conductive plates are disposed to face with each other in parallel, the capacitance therebetween is proportional to a dielectric constant of a medium interposed between the two plates and the areas of the two plates, whereas the capacitance is inversely proportional to a gap between the two plates. That is to say, the electric capacitive displacement sensor measures a displacement based on the principle that capacitance is varied when there exists a variation in the distance or overlapped area between the two plates as the two plates are moved relative to each other.
Electric capacitive displacement sensors can be classified into two types depending on their measurement methods: one type is to detect a variation in a gap between two facing plates in a vertical direction when they are moved relative to each other; and the other type is to detect a variation in an overlapped area when two facing plates are moved relative to each other in parallel (see, “Capacitive sensors”, Chapter three, Baxter, IEEE, 1997).
So far, the method using a variation in a gap (a space) between two facing plates has been preferred to measure a minute displacement since the method has an advantage in that it exhibits a high sensitivity to a displacement with a relatively simple structure. However, this method has a disadvantage in that it has a very limited measurement range for displacement since its sensitivity rapidly decreases nonlinearly as the displacement increases. Further, the gap (space) variation measurement method is very sensitive to a mechanical installation error, so extreme care must be taken for the installation of the displacement sensor. In particular, in this method, the reliability of measurement may be greatly reduced due to an abbe error, a cosine error and the like that can be caused by a discrepancy between a driving direction (e.g., a horizontal direction) and a measurement direction (e.g., a vertical direction). Also, the method is also disadvantageous with regard to effective space utilization.
Meanwhile, a non-contact type electric capacitive displacement sensor has been widely employed for a measurement of a displacement since it is not accompanied by mechanical abrasion and it can keep up with a fast driving velocity.
Recently, however, as a super-microscopic and super-precise measurement of displacement is demanded using an atomic microscope or a nano driving system, requirements for a displacement sensor has been changed. Specifically, a condition for velocity has been eased considerably, and also a mechanical installation error rather than an electric noise has risen as an important issue to be solved. As the environment in which the displacement sensor is used changes greatly, there no longer exists a necessity for an exclusive use of a non-contact type sensor.